The NINCDS-ADRDA and DSM-IV criteria are currently widely used for diagnosis of probable Alzheimer's disease (AD). These clinical criteria have a number of limitations, including lack of specificity and sensitivity in the diagnosis, and have an error rate of approximately 10% even in academic research centers. Furthermore, diagnosis based on cognitive function can only be made post symptomatically, at which time medications that may inhibit AD development or delay its progression will likely be ineffective. The imaging and biological marker diagnostic methods currently under development have additional drawbacks in terms of their need for highly specialized equipment, and specificity and sensitivity respectively, and thus may not be useful for early screening This proposal is designed to produce preliminary data for development of a biological classification of AD patients, based on high-density microarray measurement of transcribed white blood cell (leukocyte) RNA. The rationale behind this proposal is based on two sources of data: 1) Current scientific literature, in which there is growing evidence that individuals with AD exhibit immune and other responses, that can be detected at the level of altered gene expression in circulating peripheral leukocytes. Quantitation of the mRNA transcripts in leukocytes of a number of individual genes has demonstrated associations between gene expression levels and the presence of AD. 2) Preliminary results from a microarray study by the PI, investigating gene expression changes in men with schizophrenia. Initial results from this expression study have been striking: Supervised cluster analysis of peripheral leukocyte gene expression data, using transcript level measurements of thousands of genes from eight schizophrenic patients and five matched control subjects, resulted in a classification of all the subjects into their correct group. These results provide evidence to suggest that a surrogate tissue can be successfully employed for classification of a neuropsychiatric disease. These observations form the basis of the hypothesis and experimental design of this proposed study. Utilizing a similar microarray strategy, we propose to investigate our central hypothesis: that individuals suffering from AD exhibit a conserved pattern of gene expression levels in their peripheral blood leukocytes, which is distinct from the pattern of expression in peripheral blood leukocytes from control subjects. The specific aims of this proposal are; 1a. To collect blood leukocytes from thirty well characterized, AD patients and thirty well-characterized, healthy age-matched control subjects over the two-year period of this project, 1b. To employ Affymetrix microarray technology to measure simultaneously the expression levels of up to 47,000 genes transcribed in leukocytes derived from the blood of the AD patients and control subjects. 2. To analyze the leukocyte gene expression datasets collected under Specific Aim One of this proposal, using hierarchical clustering and supervised learning algorithms to identify and validate patterns of gene expression (multigene signatures) that differentiate AD subjects from normal healthy controls. The successful completion of this exploratory study will be important for providing preliminary data for larger scale NIH R01 studies, to increase subject numbers and to incorporate a longitudinal study to assess multigene expression changes during the preclinical development and progression of AD. We believe that the diagnostic technique we propose to develop may ultimately form the basis of a clinical assay that will be minimally invasive, and will have the capacity to identify AD sufferers, and in the future may also provide important pre-symptomatic and early stage diagnostic information.